A known color projection display system includes a monochromatic flat panel display device that is, in operation, sequentially illuminated with light. The modulated light pattern from the display device is then projected onto a display surface. For color projection, monochromatic light sources or selectively filtered light sources are sequentially scanned over the flat panel display device at a repetition rate sufficient that the human eye perceives a single color image. The human eye thus integrates this "color sequential" display of three separate images into a "single" image. By providing a single flat panel display device, a common optical path is provided for all colors, and convergence and misregistration errors are substantially eliminated.
To provide efficient illumination of the flat panel display device, it is preferred to split white light from a projector lamp into the three basic colors, which are simultaneously employed. Since the components are simultaneously employed, the light output from the projector lamp is efficiently employed. This technique requires that portions of the flat panel display device simultaneously present portions of pixel images for each of the colors. In order to make efficient use of the flat panel display and to avoid degradation of the resolution, each color is ideally presented as a rectangular stripe which scrolls down the panel, sequentially illuminating all regions of the flat panel display device. This technique therefore requires that pixel data for each of the respective colors be updated between the respective color stripe illumination.
In a rotating prism scanning system, the rotating prism assembly repeatedly scans the red, green, and blue bands through a pair of relay lenses, which image the spatially-separated scanning colored light bands onto a light valve panel having an array of pixels. The scanning colored light bands are separated from each other by one-third of the panel height. Each time a light band of one color leaves the bottom of the array, a corresponding light band of the same color appears at the top of the array and begins its scan. Before each colored light band passes over a respective row of pixels, the pixel image data must be loaded into the column conductors while the respective row is selected, and the pixel elements allowed to settle. Because, in this case, three different rows (or bands of rows) will be illuminated substantially simultaneously by the three different colored light bands, either three separate column conductors and drivers must be provided for each column of pixels, or the data must be provided sequentially to the column conductors at three times the video line rate.
The simultaneous use of a substantial portion of the available red, green and blue light through a single light valve panel provides optical efficiencies comparable to that of three-panel systems employing similar types of light valve panels. However, by using only a single panel, the need to mechanically converge different color images formed on different panels is eliminated, and system cost and size is reduced. Additionally, beam combining dichroic filters are not needed, which leads to additional savings. See, Peter Janssen, "A Novel Single Light Valve High Brightness HD Color Projector", Society For Information Display (SID), Technical Paper, France 1993; Shimizu, Jeffrey, "Single Panel Reflective LCD Projector", SPIE (1999).
Typically, the flat panel display device is a thin film transistor (TFT) liquid crystal display (LCD) device, having, for example, a resolution of 1280 by 1024 pixels. Since the image is projected, the display device may be relatively small, i.e., less than about 6 cm. Further, the preferred mode of operation is a reflective mode, which allows use of thinner layers of liquid crystal light modulation material and correspondingly faster response times, since the light passes through the liquid crystal twice. Display technologies other than TFT may be employed, for example known silicon on insulator LCD display devices. Further, the "projection" need not be over a large area, and, for example, a similar technology may be employed in so-called headsup displays and virtual reality goggles. See, U.S. Pat. Nos. 5,673,059 and 5,642,129, expressly incorporated herein by reference.
In order to achieve the scrolling illumination, scanning mechanisms have been proposed with moving color filters or with static color separation combined with an optical scanning mechanism like a rotating prism. The moving color filter solutions so far have been less light efficient because they tend to discard at least two thirds of the available white light to achieve individual color components. Static color separation, e.g. with dichroic mirrors, is generally much more light efficient because all color components can be used simultaneously. However, in these dichroic mirror systems, the problem is then in designing a scanning mechanism that converts the static color stripes into a useful scrolling color stripe pattern.
One known scanning mechanism is a rotating prism. It suffers, however, from low quality imaging of the color stripes, and it is generally very difficult to achieve uniform scanning for all color components in a single scanning element. Multiple scanning element systems have been proposed, employing rotating prisms (either separated or physically joined), which offer better scan-speed uniformity (for the different color light bands) and scan-speed linearity (for each light band) than the single-prism system, but are less compact. See, U.S. Pat. Nos. 5,845,981, 5,608,467, 5,548,347, 5,532,763, 5,528,318, 5,508,738, 5,416,514 and 5,410,370, expressly incorporated herein by reference.
For a scrolling scanner system, the ideal scan transformation function is: EQU x.sub.o (t)/X=(t/T+x.sub.i /X) modulo 1,
with X being the total height of input and output beam, x.sub.i being the ray height in the input beam, x.sub.o being the corresponding ray height in the output beam as a function of time, T being the frame period, and t being the time. (The modulo 1 operation returns a value between 0 and 1, equal to the non-integer fraction of the operand. It is the same as the fract () function in common computer languages.) This concept is similar to so-called "aliasing", in this case the integer portion of the function being undeterminable based on the state of the output. It is noted that only the phase of the output ray depends on the input ray height in the beam; the actual output swings always from 0 to X independently from the input ray height. This means that the scanner has to perform a different geometrical transformation for different incoming ray heights, or aberrations will result in the scrolling light band output.
In one known system, a rotating prism is provided, having a central axis of rotation and an even number of facets symmetrically disposed around the axis. A light source projects parallel beams of the three different respective colors through the prism. Central illumination rays for each of the respective color bands are propagated along a respective path that is directed at the axis of rotation. The ray paths of the outer edges of each color band are directed to converge at an angle a=(n+1/3 mb), where n is equal to any non-negative integer (i.e. 0, 1, . . . ), m is equal to 1 or 2, and b=360.degree. divided by the number of prism facets. A combination of optical components is arranged to intercept the illumination rays after their passage through the prism, and to guide and converge the illumination rays, seeking to form on the panel spaced-apart light bands having mutually parallel central illumination rays which scan across the panel as the prism is rotated. The optical elements (i.e. the prism, the lenses and the mirrors) serve to continuously focus on, and scan across, the panel, each of the light bands incident on the facets of the rotating prism. These optical elements are selected and arranged so that, as the prism rotates, the illumination rays for each of the red, green and blue light bands follows an equivalent path, from its image at a respective aperture, to the incidence of the respective light band on the light valve panel. This causes the central illumination rays for all three color bands to continuously strike the panel at the same angle of incidence (preferably orthogonal) as they are scanned across it. As the light paths leave the prism, they converge and cross before entering a lens group. After leaving respective apertures for each path, the central illumination rays for each of the light bands diverge. A set of lens groups is provided to reconverge the illumination rays to form red, green and blue images of the bands on the panel. The scan linearity of the optical system can be improved to a significant degree by making the surfaces of the revolving prism cylindrically concave. These concave surfaces refract the light, seeking to correct imperfections in the scanning function. The correction is, however, incomplete. See, U.S. Pat. Nos. 5,845,981, 5,608,467, 5,548,347, 5,532,763, 5,528,318, 5,508,738, 5,416,514 and 5,410,370, supra.
Likewise, another known attempt to scan a scrolling pattern employs a set of quasi-cylindrical lens elements disposed concentrically on a rotating disk. In this case, the output is non-telecentric and the scan is uncompensated. These quasicylindrical lens elements are employed in a liquid crystal light valve (LCLV) projection system, in which light is scanned by a train of quasi-cylindrical light bending or light reflecting elements that are sequentially interposed between a high intensity reading light source and the liquid crystal device. The quasi-cylindrical light bending elements are mounted on a circular wheel and themselves have a circular shape. The wheel is rotated to sequentially interpose the successive bending elements between the light source and the liquid crystal to cause a narrow elongated band of light to scan in synchronism with the input scan. Because of the curvature of the quasi cylindrical elements, some degree of undesirable lateral scan, orthogonal to the direction of the intended scan, is provided. Because the apparatus scans by employing an angularly deflected beam, telecentric behavior of the beam (constant angle of incidence) is lost. Telecentric behavior is desirable wherever it is important for the beam as a whole to be perpendicular to an object or image plane, such as in a liquid crystal projection system. In this system, it is considered desirable to reduce chromatic aberration, and to eliminate vertical re-trace time as the scanning shifts from one element to another in the train of quasi-cylindrical elements. The scan angle of this system is rather limited. See, U.S. Pat. No. 5,398,082 and WO 94/28672, expressly incorporated herein by reference. In a related design, the rotating lens wheel is replaced by a transparent polygonal body (i.e., a prism) mounted for rotation about an axis and driven in a continuous unidirectional rotation by a motor. Light passing through the polygonal body is twice refracted, to be displaced to an output path that is parallel to the input path. This displacement varies in magnitude as the body rotates, to effect a scanning motion of the light transmitted by the body. The angular displacement is also limited in this system. See, U.S. Pat. No. 5,428,467, expressly incorporated herein by reference. U.S. Pat. No. 5,450,219, expressly incorporated herein by reference, relates to a telecentric illumination scanning system employing a rotating polygonal mirror, which is suitable for scanning only relatively narrow light beams.
U.S. Pat. No. 5,822,025, expressly incorporated herein by reference, relates to a single light valve color projection system that enables sequential display of color. Several rotating glass plates are sequentially inserted between the light valve and the objective. These glass plates create a spatial offset of the image of light valve pixels three times per frame on the projection screen. The sets of plates are rotated by a motor with an axis parallel to the optical axis of the motor, and intersect the image beam at each image frame.
U.S. Pat. No. 5,781,251, expressly incorporated herein by reference, relates to a color single panel projector, including a mechanism for deflecting light into a plurality of directions, and a light panel for receiving the deflected light. The deflecting mechanism includes a transparent optical medium having a nonuniform thickness, in which light beams enter through a center opening of the medium and exit at areas along a side of the medium.
U.S. Pat. No. 5,490,013, expressly incorporated herein by reference, relates to a compensation plate for tilted plate optical aberrations, including astigmatism and coma.
U.S. Pat. No. 5,227,910, expressly incorporated herein by reference, relates to a laser beam scanner including a rotating prism, which receives and deflects the laser beam toward a spherical lens, which receives the laser beam and converges it. A spherical reflector then redirects and focuses the converging laser beam to a laser scan spot along a scan line. The laser beam scanner may include one or multiple prisms, wherein individual prisms may be single or multi-faceted prisms. This system is designed to eliminate across-scan error, seeking to generate a mathematically perfect scan line.
U.S. Pat. No. 5,166,820, expressly incorporated herein by reference, relates to a light scanning system having a first scanning unit for guiding the light beam in a first scanning direction, a second scanning unit including a deflection prism for guiding the light beam in a second scanning direction, and an object lens. The second scanning unit has a common optical axis with the first scanning unit and is movable in an optical direction in parallel with an optical axis or turnable around the optical axis, so that the light beam entering the second scanning unit may be deflected in any direction when exiting from the second scanning unit.
EP 0,248,204 A2 relates to a color filter wheel-based optical scanning system employing either standard color filters or diffraction grids to rapidly select a color component, allowing a single CCD image sensor array to can handle full color. The light is collimated into a narrow band of light.
U.S. Pat. No. 5,479,187 relates to an optical scanning system employing a wheel with simple planar mirror facets, suitable for scanning relatively narrow light beams. The angle of incidence of the illumination varies over the height of the panel
EP 0,749,246 A1 relates to a system having a color wheel arrangement that provides illumination pulses of color illumination components over the entire panel (in this case a digital mirror display). The illumination switches between color components more or less instantaneously over the entire panel. The panel addressing must be very fast to allow this without artifacts. Because both color illumination switch and panel addressing are not infinitely fast, some blanking is provided between color component transients. See, also U.S. Pat. No. 5,658,063.
Projection systems are also described in several U.S. Patents, including U.S. Pat. No. 4,650,296 to Koda et al for Liquid Crystal Light Valve Color Projector, U.S. Pat. No. 4,343,535 to Bleha, Jr. for Liquid Crystal Light Valve, U.S. Pat. No. 4,127,322 to Jacobsen, et al for High Brightness Full Color Image Light Valve Projection System, U.S. Pat. No. 4,191,456 to Hong, et al for Optical Block for High Brightness Full Color Video Projection System, U.S. Pat. No. 5,264,880, to Sprague et al., for method and Apparatus for projecting a Color Image, U.S. Pat. No. 5,644,357 to Cohen, et al. for Burst Driving of Single-Panel Display, and U.S. Pat. No. 5,684,504 to Verhulst et al., for Display Device, each of which is incorporated herein by reference.